It is desirable to be able to reproducibly sample populations of arthropod disease vectors (e.g., mosquitoes, sandflies, ticks and fleas), both to monitor their size and to allow analysis of the trapped arthropods to determine the presence of human diseases (e.g., malaria, dengue, Zika, West Nile Virus, Lyme disease) in the population. Many arthropod disease vectors are attracted to carbon dioxide (CO2). It is common to trap arthropods for a period of about 12 hours, for example from around sunset to around sunrise, because sunset and sunrise are times are when many disease vectors are active. The vector trap and the accompanying CO2 source must operate unattended, and the source must produce the target flow rate of CO2 over that time period. Known CO2 sources include pressure cylinders of CO2 and dry ice. However, these CO2 sources may not be available in all locations. As an alternative, CO2 can be produced at the site of the trap by mixing a carbonate or bicarbonate salt (for example, sodium bicarbonate) and an acid (for example, citric acid). Systems that operate on this principle include that described by Prohaska (U.S. Pat. No. 8,475,783 B2, Jul. 2, 2013).
Reliable operation of CO2 generators using this approach depends on combining the two reactants at a rate that will consistently produce the target amount of CO2. It is desirable that the components react to produce the CO2 rapidly, because any delay in the production of CO2 after combining the reactants may interfere with the desired steady production of CO2, and will complicate the timing of the addition of components. Consistent with this objective, portable CO2 generators that use one component in a solid form (not a liquid or a liquid solution) have used the material as a powder solid. The small particle size of a powder allows the material to dissolve rapidly in the water solution containing the other component or components. Bicarbonate or carbonate salts are commercially available as powder solids so that no additional processing is required. Additional processing is undesirable because it adds to the cost of the consumable material, and requires the manufacturer of the consumable supplies to carry an inventory of a processed material. Consistent with these considerations, systems using on-site generation of CO2 for vector trapping use mixing of a powder solid with a water solution. For example, see Prohaska (2013).
The prior art teaches that it is important to use powdered salts so that an even and measured CO2 generation rate can be achieved, while at the same time identifying problems associated with the actual use of powders in unattended systems. For example, in U.S. Pat. No. 6,920,716, Kollars et al. disclose a non-electrical carbon dioxide generating arthropod trap. In this disclosure the combination of baking soda and vinegar is used to generate carbon dioxide gas with the optional addition of urea, lactic acid, and ammonia as further attractants. In this device the dry sodium bicarbonate powder (baking soda) is placed in a separate reactor container and aqueous solution of acetic acid (vinegar) is dripped into the reactor container to produce carbon dioxide. However, this method of mixing the reactants leads to powder caking and inconsistent gas flow rates.
Prohaska et al. teaches a potential solution to the problem of using powdered salts. U.S. Pat. No. 8,475,783 B2 teaches a device for generating carbon dioxide as an attractant for biting arthropods in combination with a trap, comprising: a reaction chamber charged with an aqueous acid solution when in use; a gas outlet from the reaction chamber connecting between the reaction chamber and the trap; a feeder reservoir containing a powder when in use, said powder comprising a bicarbonate salt; and means for controllably adding the powder from the feeder reservoir to the reaction chamber; whereby carbon dioxide is generated in the reaction chamber, passed through the outlet and into the trap. Further disclosed is an improved arthropod trap for catching biting arthropods assisted by the evolution of carbon dioxide, the improvement comprising: a reaction chamber charged with an aqueous acid solution when in use; a gas outlet from the reaction chamber connecting between the reaction chamber and the trap; a feeder reservoir containing a powder when in use, said powder comprising a bicarbonate salt; and means for controllably adding the powder from the feeder reservoir to the reaction chamber; whereby carbon dioxide is generated in the reaction chamber, passed through the outlet and through or into the trap.
Prohaska further admits potential problems when operating solid powder fed traps in humid conditions. He teaches that the powder comprising a bicarbonate salt may further comprise additives that permit flow of the powder in humid conditions. These include anti caking agents such as silicon dioxide, aluminum oxide, boron nitride, calcium chloride, magnesium sulfate, calcium bentonite, sodium bentonite, sodium alumino-silicate, magnesium carbonate, calcium silicate, tricalcium phosphate, talc, kaolin, starch, cellulose or combinations thereof. In addition to adding anti-caking agents to the powder comprising a bicarbonate salt, Prohaska states that it may be necessary to heat the powder before it is released into the reaction vessel to a temperature of 35° C. immediately prior to use to prevent caking in humid conditions.
The arthropod traps of the prior art suffer from at least one of the following limitations: powdered reagents are required to adequately meter the solid into the water solution and to result in controllable CO2 generation rates, they suffer from poor performance in humid conditions due to caking of powders and poor solid metering control.